This application claims the priority of German patent document 199 28 102.5, filed Jun. 19, 1999, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a vehicle having an internal-combustion engine, a starter battery or buffer battery and a fuel cell system to supply electric consuming devices of the vehicle, and to a method for operating such a vehicle.
In a known vehicle which is driven by an internal-combustion engine, a fuel cell system supplies energy to the electric consuming devices of the internal-combustion engine, and can be activated independently of the operation of the internal-combustion engine. The fuel cell system also supplies the internal-combustion engine with electric energy. A battery connected with the fuel cell system outputs current when the internal-combustion engine is started, and covers brief peak loads (German Patent Document DE 197 03 171 A1). In this vehicle, the fuel cell is supplied with hydrogen, either from a tank or indirectly by reforming liquid or gaseous fuels, by way of a controlled throttle. Reaction air is supplied to the fuel cell by way of a compressor, with a throttle connected on the output side.
A motor vehicle driven by an internal-combustion engine is also known which, instead of a dynamo, has a fuel cell system that provides the electric energy for operating consuming devices independently of the rotational speed of the internal-combustion engine. In this motor vehicle, the internal-combustion engine is supplied with liquid fuel. Hydrogen is separated from the liquid fuel for the operation of the fuel cell system. Only a portion of the hydrogen is separated from the fuel. The remaining fuel constituents are further utilized in the internal-combustion engine; that is, gaseous constituents are feed to the internal-combustion engine directly or by way of a mixture formation device and liquid constituents are returned into the fuel tank. An intermediate H.sub.2 storage device provided in the supply line supplies the fuel cell at the time of the starting of the vehicle. A battery is provided for starting the fuel cell system and/or a separating device for separating the hydrogen from the liquid fuel. (German Patent Document DE 195 23 109 A1)
Another fuel cell system which has been suggested for use with an internal-combustion engine supplied with liquid fuel from a tank, generates electric energy during the operation of the internal-combustion engine, at least for the electric devices provided for the operation of the internal-combustion engine, and is connected with a buffer battery. In this system, exhaust gases or heating gases, which are generated as the result of the combustion of the liquid fuel, are guided through at least one heat exchanger in the fuel cell system until an operating temperature sufficient for operation of the fuel cell system is reached. (German Patent Document 199 13 795.1)
Finally, a vehicle has also been suggested which is propelled by an internal-combustion engine drive and by a fuel cell drive, with liquid fuel being used to supply both such drives. The internal-combustion engine drive is used to start the vehicle, and to permit its immediate movement thereafter, as well as to heat the fuel cell system to an operating temperature. The fuel cell is started when it reaches the operating temperature, and thereafter an electric drive motor, (fed by the fuel cell system) generates torque for movement of the vehicle, either alone or together with the internal-combustion engine. (German Patent Document 199 13 794.3)
Due to an increase in the number of electric consuming devices in trucks and passenger cars, difficulties arise when generators are used for the power supply. At approximately 15%, the efficiency of dynamos is unfavorable. Moreover, when the vehicle is stopped, the battery can only be loaded briefly in order to avoid impairing its starting capacity. By using fuel cells, which have a significantly higher efficiency than generators, and can be operated independently of the operation of the internal-combustion engine, the power supply while the vehicle is stopped can be improved, even for vehicles with several consuming devices.
In the future, pollutant emissions of motor vehicles must also still be drastically reduced. The reduction of, for example, nitrogen oxides in exhaust gases of conventional internal-combustion engines, such as diesel engines, is performed by means of continuous exhaust gas catalysts which have a poor cold-starting behavior. As the result, 90% of the nitrogen oxides produced during a driving cycle are caused by the poor cold-starting behavior of the currently used exhaust gas catalysts.
One object of the invention is to provide a vehicle that is driven by an internal-combustion engine, and has a high-capacity and high-efficiency power supply unit.
Another object of the invention is to provide such a vehicle in which the pollutant emission is greatly reduced in a relatively simple manner, including in the cold starting phase.
Finally, still another object of the invention is to provide a method for operating such a vehicle.
These and other objects and advantages are achieved by the vehicle according to the invention, in which the fuel cell system has an electrically heatable, autothermal reforming reactor, to which air and water can be fed by assemblies in the vehicle and hydrocarbons can be fed from the fuel tank, for partial oxidation and vapor reforming. A shift reactor is connected behind the reforming reactor for reducing the carbon monoxide fraction On its output side, the shift reactor is connected on the one hand to the anode-side input of a fuel cell, via a valve which is penetrable after the operating temperature has been reached in the shift reactor, and is connected on the other hand to the driving internal-combustion engine, via a bypass of the fuel cell, which can be blocked when the operating temperature has been reached in the shift reactor. When the reforming reactor is heated electrically, hydrogen-containing gas, which will be available within a very short time, is burned in the internal-combustion engine with the hydrocarbons delivered from the fuel tank. Pollutant emissions are reduced significantly by the metered addition of hydrogen during the initial operating minutes of the internal-combustion engine, because a cleaner combustion of the hydrocarbons takes place as the result of the hydrogen.
"Autothermal" means in this case that the heat required for the process is provided in the educt itself by the partial oxidation. Heat will be supplied by electric heating only when the engine is started. The hydrogen-containing gas flows through the shift reactor and, in the process, heats it to its operating temperature. When the operating temperature in the shift reactor has been reached, the hydrogen-containing gas is guided from the shift reactor into the fuel cell by opening the valve connected in front of the fuel cell and blocking the bypass. The hydrogen will then be oxidized at the anode of the fuel cell and the oxygen will be reduced at the cathode, so that the fuel cell supplies electric power for the electric consuming devices of the motor vehicle.
A pollutant reduction in the exhaust gases is achieved by the utilization of the hydrogen generated for the fuel cell during a time period in which the fuel cell has not yet started its operation because of a warm-up phase. It is unnecessary to provide a hydrogen tank, which is hard to integrate in the vehicle because of its volume and weight, for reducing the pollutants.
Because of the intensive use of energy for which the battery would have to be designed, the generation of hydrogen by electrolysis is not practical.
In a preferred embodiment, the residual anode gas output of the fuel cell is connected with a pipe extending to the driving internal-combustion engine, and the bypass also leads into the same pipe. In this embodiment, the portion of the hydrogen not consumed in the fuel cell, and other burnable constituents in the anode exhaust gas, are burned in the engine. Also after the cold starting phase, hydrogen is still supplied to the fuel taken from the tank, whereby, also when the exhaust gas catalyst has reached its operating temperature, a greater pollutant reduction is achieved in the exhaust gases in comparison to vehicles which do not have the system according to the invention. The cathode-side output of the fuel cell is preferably connected with a water tank by way of a separator; and the reforming reactor is also connected to the water tank. In this case, water required for the reforming process is recovered from the cathode exhaust gas.
In particular, an air supply input of the reforming reactor is connected with the air supply device of the internal-combustion engine. The air for the internal-combustion engine can be generated, for example, by a compressor which is connected on the output side with the internal-combustion engine or the device for the mixture formation and the reforming reactor.
In another preferred embodiment, the air ratio of the reforming process is set to be less than 1, and the ratio of water to carbon is greater than 0. The hydrogen is generated by means of an understoichiometric combustion. The air ratios are preferably in the range of from 0.2 to 0.6, 0.3 being optimal.
The objects of the invention are also achieved by a method for operating a vehicle having an internal-combustion engine, a starter battery or a buffer battery and a fuel cell system for supplying energy to electric consuming devices of the vehicle equipped. According to the invention, after the driving internal-combustion engine is started, a hydrogen-containing gas is generated from the fed educts air, water and a fuel of hydrocarbons in an autothermal reforming reactor electrically which is heated to the operating temperature. The hydrogen containing gas is supplied to the internal-combustion engine together with the hydrocarbon fuel by way of a shift reactor. After the shift reactor is heated to its operating temperature for reducing the carbon monoxide fraction in the hydrogen-containing gas, the hydrogen-containing gas is no longer fed directly to the internal-combustion engine. Rather, it is fed to a fuel cell whose anode exhaust gases, together with the hydrocarbon fuel, are supplied to the internal-combustion engine.
The reforming reactor is electrically heated to its operating temperature in a few seconds; this, the hydrogen-containing gas is available for admixture with the fuels virtually immediately after the internal-combustion engine is started. Because of the resulting improved combustion in the engine, the pollutant emission is significantly reduced, even in the cold starting phase. After the fuel cell has started its operation, the anode exhaust gas, which still contains hydrogen is supplied to the internal-combustion engine together with the fuel, to reduce pollutant emissions. By reducing pollutant emissions, it is possible to design the internal-combustion engine for higher combustion temperatures, which increases the efficiency of the engine while maintaining nitrogen oxide emissions (which are greater at higher combustion temperatures) at acceptable values. Simultaneously, the fuel cell also supplies the current for a larger number of consuming devices which may have a high power requirement.
Water for the reforming reactor is preferably obtained by separation from the cathode exhaust gas and/or from the exhaust gases of the internal-combustion engine, so that it is no longer necessary Lo carry a large water supply in the vehicle. Moreover, it is expedient for the air for the reforming reactor and the fuel cell to be branched off from the air flow for the driving internal-combustion engine, eliminating the need for separate units for compressing the air required by the reforming reactor and the fuel cell. In particular, the hydrogen-containing gas is generated in the reforming reactor by the partial oxidation and vapor reforming with an understoichiometric combustion at an air ratio of less than 1, the ratio of water to carbon in the fuel being set to be more than zero in order to avoid a soot formation.
In another advantageous embodiment, liquid fuel is used to supply the reforming reactor and the internal-combustion engine. In the case of diesel oil, the ratio of water to carbon is set to a value more than 1.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.